Electric switch



Patented Nov.v 3, 1936 ELECTRIC SWITCH Lynn H. Matthias, Shorewood, Wis., assignor to Allen-Bradley Company, Milwaukee, Wis., a. corporation of Wisconsin Application September 15, 1934, Serial No. 744,131

Claims.

This invention relates to electricswitches and refers more particularly to switches of the type known as starting switches, one form of which is illustrated in the copending application of Gus- 5, tavl O. Wilnis and Albert J. Dawe, Serial No. 710,656, filed February 10th, 1934, now Patent No. 1,981,534, issued November 20, 193i.

In switches of this general type, there is usually a set of stationary contacts engageable by a set i@ of complementary movable contacts. Upon engagement of the contacts and the owoi current therethrough, the inherent contact resistance causes the generation of heat.

Obviously, it is desirable to keep this contact temperature rise as low as possible.

This is the broad object of the present invenn tion.

Heretofore, it was assumed that the following physical characteristics of the contacts deter- 2@ mined the contact temperature rise:

(l) The material of which the contacts were composed.

(2) The contact pressure. That is, the pressure with which the contacts were held closed.

(3) The shape of the contacts, which controlled the mass of material proximate to the point of contact and thereby governed the ability of the contacts to absorb generated heat.

It was discovered, however, that these physical characteristics of the contacts alone did not determine the contact temperature rise. In .attempting to overcome the objectionable contact temperature rise noted, contacts of many dierent kinds ci metal were tried, but without success. Increasing the contact pressure -to the maximum permissible as determined by the strength of the operating magnet, and varying the shape of the contacting surfaces also did not pre vent the contacts from running hot.

40 After considerable research, a further and very important cause oi objectionable contact ternperature was found to reside in the fact that the movable contacts vibrated.

' Such vibration was of course accompanied by a substantial increase in contact resistance and a consequent rise in contact temperature.

'It is therefore a further object of this inven tion to overcome this objectionable vibration of the movable contacts.

. To attain this end, three different solutions was a fiat surface for the stationary contact and a rounded or spherical surface for the movable contact. This shape adorded the greatest mass surrounding the point of actual contact and thus assured the optimum of heat dissipation through 5 absorption; but it permitted the movable contact to rock or roll about on the stationary contact.

A fiat surface for both contacts theoretically would prevent rocking or rolling and if accompanied by suicient contact pressure would pre- 10 vent vibration, but the dimculty of lining up the contacts so that their meeting faces would have the desired at engagement, especially in a construction in which the two movable contacts are carried by a common contacter, and the almost l5 certain probability of the, contacts engaging at their edges where the lack ci suicient proximate mass would cause them to weld, precluded solutionof the problem in this manner.

The second solution suggested to overcome the 2@ vibration was increased contact pressure. Exn periments, however, showed that even with the4 strongest springs permissible as determined by the strength of the operating magnet, the vibration still persisted. 25

'I'he third solution was to provide some mechanical means engageable with the movable contact to stop its vibration.

This is the speciiic object oi this invention,

With the above and other objects in view which 3@ will appear as the description proceeds, this invention resides in the novel construction, ccmbination and arrangement of parts substantiahy as hereinafter described and more particularly defined by the appended claims, it being under- 35,

stood that such changes in the precise embodiment of the herein disclosed invention may be made as come within the scope of the claims.

The accompanying drawing illustrates one com pleteeriample of the physical embodiment oi the c@ invention constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:

Figure 1 is a section view through the contacts of a switch of the type illustrated in the afore- 45 said copending application showing the movable contacts disengaged from their complementary stationary contacts;

Figure 2 is a. section View taken through Figure 1 on the plane of the line 2 2; and 5o Figure 3 is a magnified diagrammatic view to illustrate the theory which has been advanced as explanatory of the cause of the vibration.

Referring now more particularly to the accompanying drawing, numeral 5 designates a contact 55 mounting head preferably formed of insulating material and provided with pockets B on its underside which form arc suppressing chambers to the ceilings of which the stationary contacts 'I are secured, only one set of stationary contacts being shown.

'I'he contacts 'l are adapted to be electrically bridged by a contactor 8 carrying rounded contacts 9 at its extremities for engagement with the flat surfaced contacts 1.

The contactor 8 is mounted on an actuator I0 arranged to be moved towardand, from the stationary contacts to carry the movable contacts into and out of engagement therewith.

To afford contact pressure, an expansive spring II is coiled about a post I2 by which the contactor is mounted on the actuator, the spring being confined between the contactor and the actuator I to be compressed upon closure of the switch.

As pointed .out hereinbefore, it was found that the movable contacts vibrated when in their closed position, thus increasing the effective contact resistance and causing the contact temperature to rise abnormally. In the construction illustrated, this vibration amounted to a rocking motion on the part of the movable contactor and the contacts xed thereon about an axis longitudinal to the contactor and passing through the points of contact between the movable and stationary contacts.

To overcome this rocking motion, a spring finger I3 frictionally engages the contactor. This spring finger isflxed in any suitable manner to the actuator I0, the mounting head 5, or any other support. If carried by the actuator I0 as shown it may be mounted thereon by having a portion I4 thereof confined between the spring I I and the actuator. The outer free end. of the spring nger I3 bears against the side of the contactor as illustrated and through this frictional engagement effectively precludes the objectionable rocking motion or vibration.

The explanation of the vibration of the contacts is thought to lie in the thermal expansion of the contact material. To illustrate this theory reference is made to Figure 3 which represents a greatly magnified view in cross section through a pair of engaged contacts.

When engaged as illustrated current passes from one contact t`o the other through the small contact area at a point A. The resistance at this point of contact generates heat. The heat so generated is absorbed by the adjacent contact material and the surrounding air. 'I'he heat absorbed by the contact material causes the contact material to expandand inasmuch as the maximum heat is absorbed at the point of contact, the thermal expansion is greatest at this point and as a result microscopically small hills form on the contacts at their points of engagement.

Except in very rare occasions, the point of contact A is displaced from the center line of the force applied by the spring. Hence, the formation of the hills causes the lower movable contact to roll from its position indicated in full lines to a position which may be represented by the dotted lines and at which the point of contact is defined by the engagement of points B and B' on the movable and stationary contacts respectively.

The movable contact rolls or shifts in response to th thermal expansion at the point of initial contact until the force exercised by the thermal expansion hills plus the kinetic energy of the moving contact equals the spring force tending to return the contact to its initial position. When this limit of motion has been reached and the points B and B' are in engagement the direction of motion reverses and the oscillation continues.

In view of the fact that it requires but a microscopic hill or in other words a very slight force to induce the rocking motion, it follows that very little friction or retarding force is required to prevent the objectionable vibration, and this is admirably accomplished by the addition of the slight spring tension on the side of the contactor as illustrated or directly on the side of the movable contacts.

The explanation advanced herein for the cause of the vibration of the movable contacts is to be understood as merely a theory. However, regardless of whether this theory correctly explains the vibration at the contacts, the fact that vibration exists and that the vibration is the cause of the objectionable contact temperature rise, has been discovered; and this discovery has enabled the solution of a particularly perplexing problem.

What I claim as my invention is:

1. In combination with cooperating electric switch contacts having engaging surfaces, one of which is substantially convex so that when envgaged one of the contacts is apt to rock on the other, means acting on said contact to prevent objectionable rocking thereof while enabling said contact to adjust itself with respect to the other contact.

2. In combination with a pair of complementary switch contacts, one of which is movable and one of the contacting surfaces of vwhich is substanvtialiy convex so that one contact is free to rock on the other, friction means acting on the movable contact to hold the same against rocking movement on the other contact.

3. In an electricY switch, cooperating contacts having meeting faces at least one of which is convex so that rocking of one contact on the other is apt to occur when the contacts are engaged and current is flowing from one to the other, spring means to provide contact pressure, and means for offering resistance to a tendency of the contacts to rock without interfering with the normal function of the spring means.

4. In an electric switch, cooperating contacts, means mounting the contacts for engagement and disengagement, one of the meeting faces of the contacts being substantially convex and the manner of mounting the contacts being such that one contact is apt to rock on the other when the contacts are engaged and current isfiowing from one to the other, and means offering resistance to a tendency of the contacts to rock without interfering with their normal engagement and disengagement.

5. In an electric switch, cooperating contacts adapted to be engaged and held in engagement under spring tension the meeting face of one contact being substantially convex, the mounting for the contacts and the shape of their meetstant spring pressure as long as they are engaged, the manner of mounting the contacts and the shape of their meeting faces being such that one contact is apt to rock on the other when the contacts are engaged and current is flowing from one to the other, and means for oiering resistance to a tendency of the contacts to rock without interfering with the normal functioning of the springmeans and engagement and disengagement of the contacts.

7. In an electric switch, a stationary contact, a movable contact engageable therewith, an actuatorr for carrying the movable contact to and from engagement with the stationary contact including a springarranged to press the movable contact toward the stationary contact with constant pressure as long as the contacts are engaged, the meeting face of one of the contacts being convex to have a substantially point contact with the meeting face of the other contact and the manner in which the movable contact is held by the spring being such that the movable contact is apt to rock on the stationary contact when the contacts are engaged and current is flowing from one to the other, and means for introducing a frictional force so applied to the movable contact as to resist a tendency on the part of the movable contact to rock.

8. In an electric switch, a stationary contact, -a movable contact engageable therewith, an actuator for carrying the movable contact to and from engagement with the stationary contact including a spring arranged to press the movable contact toward the stationary contact with constant pressure as long as the contacts are engaged, the meeting face of one of the contacts being convex to have a substantially point contact with the meeting face of the other contact and `the manner in which the movable contact is held by the spring being such that the movable contact is apt to rock on the stationary contact when the contacts are engaged and current is flowing from one to the other, and means acting on the movable contact in such a manner as to resist a tendency of the same to rock on the stationary contact without interfering with the normal engagement and disengagement of the movable contact with the stationary contact.

9. In an electric switch, two spaced stationary contacts, a bridging bar constituting a movable contacter movable to and from a closed position electrically bridging the stationary contacts, actuating means for the bridging bar including a spring to provide contact pressure and so mounting the bar as to allow the bar to adjust itself to the stationary contacts whereby the bar is free to rock on its longitudinal axis, and means onering resistance to a tendency of the bar to rock on its longitudinal axis without effectively interfering with the normal function of the spring.

10. In an electric switch, a plurality of spaced stationary contacts, a plurality of correspondingly spaced movable contacts, a bar rigidly connecting the movable contacts and electrically bridging the same, actuating means mounting the bar for movement toward and from the stationary contacts, said actuating means including resilient means for providing contact pressure and so mounting the bar that the bar is free to rock on its longitudinal axis, and means for restraining such rocking movement of the `bar without effectively interfering with the normal function of the resilient means.

LYNN H. MATTHIAS. 

